Color television, burst separation system



R. N. RHODES Nov. 10, 1959 I COLOR TELEVISION, BURST SEPARATION SYSTEM I2 Sheets-Sheet 1 Filed oat. 11, 1955 v 11 15 14 T r f" LUM/IV A/L'i @F175' W050 5/6? 057. fiMPL. 1 CAWA/A/EL :7- 15 c/woi/mml 187 's/mm 6010 SY/VC EVA/afar: 05, 100, smear emu. I,

f 1! meme: 27

INVENTOR.

Romain/319110008 QFQZM/ R. N. RHODES 2,912,489 COLOR TELEVISION, BURSTSEPARATION SYSTEM 2 Sheets-Sheet 2 Nov. 10, 1959 Filed Oct. 11, 19552,912,489 Patented Nov. 10, 1959 Figure 4 illustrates in block andschematic form a porof a color television receiver involving amodificaicgi 9' ti an Ofthe mbodiment of the present invention illus- LH g t r M trated in Figure 1. COLOR TEIJEVISI ON, BURST SEPARATION b inFigure 1 a portion of a conventional television.re SYS M 1 ceive'r is'illustrated in simplified block form. The receiver a i cludes the usualRF, IF and video detector sections, astime?mistreatmentawe?" r pr e by hla k e Tha r r I 4 r, I V v video signal output of the video detector isapplied to a Application October 11, 1955, Serial No. 539,843 10 videoamplifier 13, as well as to suitable circuits for f separating thedeflection synchronizing puises from the 1 Clam (CL I'm-54) appliedcomposite video signal. An output of video amplifier 13 is applied via asuitable luminance signal 7 H r channel 14 to the fred, greenl and bluecathodes, I The present invention relates generally to color tele- 15 ineommon', of a color kirrescope 1 6; an output of video visionreceivers,and in particularto an improved'system amplifier 13 is alsoapplied to a bandpass amplifier 17, for separatihg the colorsynchronizing burst from a re-' which is provided with a passbandappropriate to passage ce'ivedeomposite color picture signal insuchreceiver's of the color sub'carrier and its 'sidebands that comprise ltha s heretofore been customary to efiect burst s'epathe chrominancesignal portion of the composite video ration in a color receiver byapplying the output of t'he signal. The output of bandpass amplifier 17is applied receivers chrom'inance bandpass amplifier to a tube which tothe receivers color demodulators 18 for recovery of is gated intoconduction by a kick'-back pulse derived respective color-differencesignals to be applied tothe from the receivers horizontaldefiectionsystem. It has appropriate control grids of the colorkinescope 16.

been found desirable to providesome means for fixing the The output orthe bandpass amplifier 17 is also applied level at theinputof the gatedburst separator tube to a burst separator 19, embodying the principlesof the insure proper amplification otthe applied burst depresentinvention. The burst. separator 19 includes an spite changes in theamplitude of the gatin g pulse (which amplifier tube 21-, illustrativelyshown as a pentode, the may occur due to load variations on thereceivers'high plate circuit of the tube 21 comprising a parallelresonant voltage supply). Thus, it has been proposed that a diodecircuit 23 tuned to the color s'ub'carrier frequency. The level setterbe employed in the gating pulse input circuit inductance element 23L ofthe resonant circuit 23 serves to the D.-C. level so that positive peaksof the burst as the primary of a transformer which couples the sepadonot draw grid current and negative peaks of burst do rated burst outputof tube 21 to a phase detector 25,

not go beyond cut-01f. The present invention is di ected which developsa control voltage for automatic frequency toward an arrangement foraccomplishing these purposes control of the receivers 3.58 the.oscillator 27. The frewithout the necessity of providingthe additionaldiode 'quency control of oscillator 2'7 is effected in the usual andwith the advantage of providing improved noise manner by a rcactancetube circuit 29, responsive to the immunity. control voltage output ofthe phase detector 25.

In accordance with an embodiment of the present in- The burst separator19 operates on familiar gated ventio'n', separated horizontalsynchronizing pulses are amplifier principles, a flyback pulse derivedfrom the horiapplied to the control grid of the burst separator tube 40zontal output transformer (not specifically illustrated) of "in additionto the gating fiyback pulses, the sync pulses the horizontal deflectioncircuits 31 being applied to the being of sufficient amplitude to drawtgrid current and signal input circuit of the amplifier tube 21, thatis, the thus efiect the desired level setting. By driving the concontrolgrid 22 of the tube 21 in addition to the chromit'rol grid togridcurrent during the portion of the gating na'nce signal output ofbandpass amplifier 17. The cirinterval immediately preceding the burst,noise compoc'uit parameters are adjusted such that the tube 21 is gatednent's occurring on the chrominance signal during this into conductiononly during the recurring peaks of the pre-burst period aresubstantially suppressed by the grid flyback pulse waveform derived fromdeflection circurrent and a significant improvement of thesignal-tocuits 31. noise ratio of the separated burst is therebyachieved. In accordance with the principles of the present inven-A'ccordingly, a primary object of the present invention tion, however,an additional pulse waveform is also ap- 'isfto provide a colortelevision receiver with an improved plied to the control grid 22 oftube 21. This additional arrangement for separating the colorsynchronizing burst pulse waveform comprises a train of separated syncfrom the received composite color picture signal. 7 pulses (of positivepolarity) derived from the output of An additional object of the presentinvention is to the sync separator circuits 15. To appreciate the ad-'provide a novel and improved burst gating circuit wherein vantagesobtained by the additional application of sepaadequate burstamplification without distortion is substanrated sync pulses to thegated amplifier control grid 22, tially assured. reference may now bemade to the Waveforms illustrated A further object of the presentinvention is to provide in Figures 2a, 2b, 2c and 2d, and in Figures 3aand 3b. 'a novel burst separator circuit with substantially im- Figure2a illustrates the appearance of the composite proved noise immunity. 6Owaveform at control grid 22 in the absence of applica- Other objects andadvantages of the present invention tion of the separated sync pulsesthereto. The subcarrier will occur to those skilled in 'the art after areading of burst b isshown superimposed upon the fiyback pulse thefollowing detailed description and an inspection of fif In addition,noise components n, of a frequency the accompanying drawings in which:in the region of the subcarrier frequency, are assumed to Figure 1illustrates in block and schematic form a be presentin the output of thechrominance signal bandportion of a color television receiver includinga burst pass amplifier, andare shown as superimposed upon the separatorembodying the principles of the present composite flyback pulse-burstwaveform. The axis labeled invention. 7 co in Figure 2arepresentscut-ofi potential. It willfbe Figures 2a, 2b, 2c, 2a, 3a and3billustrate graphically that the delayed time location of theburstrelasignal waveforms which aid in an explanation of the tive to the peakof the gating fiyback pulses requires an operation and advantages ofembodiments of the present effective gate width g which is substantiallylonger than invention. the duration of the burst. It will also beobserved that the noise components occurring in the output of bandpassamplifier 17 throughout this relatively wide interval therefore appearin the output of the gated tube 21, as

illustrated by the output waveform of Figure 2b.

Figures 20 and 2d illustrate the effect on the input Waveform to thegated tube 21 of variations in the amplitude of the gating flyback pulsein the absence of level setting means. It will be seen in Figure 20 thatan increase in flyback pulse amplitude may result in the positive peaksof the burst driving tube 21 into grid current. Figure 2d shows that adecrease in flyback pulse amplitude may result in the negative peaks ofthe burst driving the tube 21 beyond cut-ofiF.

Since it is normally desired that amplification of the burst in theseparating operation be as high as possible, reduction of the burstinput amplitude to avoid such burst distortions is not a satisfactorysolution. Use of a diode level setter in the burst separator inputcircuit is one possible solution, whereby maximum burst amplification inthe separating operation may be achieved without danger of burstdistortion despite flyback pulse amplitude variations.

The present invention, however, provides a more economical manner ofeffecting such level setting, with the added advantage of eliminating toa substantial extent the noise components which might otherwise pass tothe output of the burst separator during the pre-burst portion of theburst gating interval. Figure 3a illustrates the composite waveformappearing at control grid 22 when separated sync pulses of positivepolarity are applied thereto in accordance with the embodiment of theinvention illustrated in Figure 1. As shown in Figure 3a, each positivesync pulse p occurring during the preburst portion of the gatinginterval, is of sutficient amplitude (suitably greater than theamplitude of the bursts positive peaks) that its peak regularly drivestube 21 to grid current, effectively setting the D.-C. level of thecomposite waveform such that undistorted amplification of the burst isassured. As shown by the output waveform of Figure 3b, a result of theflow of grid current during the pre-burst application of the positivesync pulse is a smearing and substantial elimination of thecoincidentally occurring noise components n accompanying the burst inthe output of the bandpass amplifier 17.

Figure 4 illustrates application of the principles of the presentinvention to modification of a color television receiver of the RCA21CT66lU type. Detailed information concerning this type of receiver maybe readily obtained from the RCA Victor Service Data Pamphlet designated1955 No. T5. While only some of the receiver sections have beenillustrated in Figure 4, and and many of these only in simplified blockform, certain sections such as the second detector, first videoamplifier, chroma bandpass amplifier and second sync amplifier have beenillustrated in schematic detail for ease in understanding of the detailsof the necessary modification. Where feasible, the same referencenumerals have been used to designate corresponding elements in Figures land 4.

In Figure 4, it will be noted that composite video signals are derivedfrom both plate and cathode of the first video amplifier 41. The signalsderived from the cathode are applied to the luminance signal channelwhich feeds the luminance signal in common to the respective red, blueand green cathodes of the electron gun structure of the receivers colorkinescope. The signals appearing at the plate of amplifier 41 areapplied to the chrominance signal channel 47, which includes, incascade, a color take-off transformer 49, a bandpass amplifier 51, abandpass transformer 53 and a demodulator driver 55. The demodulatordriver 55 supplies the chrominance signal output of the chrominancesignal channel 47 to the receivers RY and G-Y demodulators, the outputsof which are fed to respectively appropriate control grids of theelectron gun structure of the color kinescope, and are also suitablycombined in a matrix to form a BY signal, which is separately amplifiedfor application to the remaining color kinescope control grid.

The 3.58 mc. oscillations required for operation of the demodulators areobtained from the oscillator 27. Frequency control of oscillator 27 iseffected by the reactance tube circuit 29 responsive to the errorvoltage out put of phase detector 25, which compares the output ofoscillator 27 with the color synchronizing bursts of subcarrierfrequency passed to it by the burst separator 119.

The burst separator 119, embodying the principles of the presentinvention, corresponds generally in principle of operation to the burstseparator 19 of Figure 1, but differs in circuit details to be noted. Itwill be observed that the burst separator tube 121 takes the form of atriode, with the output of bandpass amplifier 51 applied to the cathode121C thereof. Flyback pulses, derived from a horizontal outputtransformer secondary winding 132, are applied via a resistive dividingnetwork 134136 and a coupling capacitor 138 to the control grid 121G ofthe separator tube 121. Also applied to the control grid 1216, viaresistor 14-1! and capacitor 142, are separated and amplified syncpulses of positive polarity derived from the plate of the second syncamplifier 158 in the receivers sync signal channel 15. A resistor 144connects the control grid 121G to a suitable source of negative biaspotential.

Although, in the embodiment of Figure 4, the output of bandpassamplifier 51 is applied to a different beam control electrode (i.e.cathode 121C) than that to which the flyback and sync pulses areapplied, it will be readily appreciated that the principles of operationand the resultant advantages are generally the same for this embodimentas for the embodiment of Figure l, as previously explained with the aidof the waveforms of Figures 2 and 3, since the cathode 121C is a part ofthe input circuit to the tube 121. Comparison of the circuits of Figure4 with the schematic diagrams illustrated in the aforementioned servicedata pamphlet will illustrate the simplicity with which a conventionalcolor television receiver may be modified to enjoy the advantages of thepresent invention. it will also be appreciated that, while Figures 1 and4 have illustrated two specific forms of apparatus for practicing thepresent invention, a number of other circuit modifications are feasiblein providing such apparatus.

Having thus described the invention, what is claimed In a colortelevision receiver provided with a source of composite signalsincluding recurring color synchronization bursts of oscillations, asource of horizontal flyback pulses, and a source of deflectionsynchronizing pulses, burst separating apparatus comprising an electrondischarge device having a signal input circuit, means for applying saidcomposite signals from said first-named source to the signal inputcircuit of said electron discharge device, means for applying flybackpulses from said second-named source to said input circuit forselectively gating said electron discharge device into conduction, andmeans for applying synchronizing pulses from said third-named source tosaid input circuit to control the DC. level of said gating flybackpulses.

References Cited in the file of this patent UNITED STATES PATENTS BartonApr. 29, 1952

